Method for designing overlay targets and method and system for measuring overlay error using the same

ABSTRACT

A method for designing an overlay target comprises selecting a plurality of overlay target pairs having different overlay errors or offsets, calculating a deviation of the simulated diffraction spectrum for each overlay target pair, selecting a plurality of sensitive overlay target pairs by taking the deviation of the simulated diffraction spectrum into consideration, selecting an objective overlay target pair from the sensitive overlay target pairs by taking the influence of the structural parameters to the simulated diffraction spectrum into consideration, and designing the overlay target pair based on the structural parameter of the objective overlay target pair.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a system and method formeasuring an overlay error, and more particularly, to a method formeasuring an overlay error by matching the deviations of the diffractionspectrum of overlay target pairs.

2. Description of the Related Art

Continuing improvements in semiconductor process technology haveincreased the accuracy required for overlay measurement. According tothe ITRS (International Technology Roadmap for Semiconductor) theoverlay tolerance in the semiconductor process is about one-sixth ofline width, and the corresponding metrology error budget is only aboutone-sixtieth of line width. Therefore, due to diffraction limitationsand proximity effects, image-based overlay metrology cannot fulfill theaccuracy requirements for the next-generation structural parametermeasurement. Diffraction-based metrology, which is different fromimage-based overlay metrology and is not influenced by the proximityeffects, has high repeatability and reproducibility characteristics andwill become an important overlay measurement technology. An overlaymeasurement system using diffraction-based metrology technology iscomprised of a scatterometer and a program for matching analysis, andthe analysis technique can be a theoretical model-based method or anempirical model-based method. The spectrum of the theoreticalmodel-based method is calculated using a theory such as RCWT (RigorousCoupled Wave Theory) or FDTD (Finite Difference Time Domain) and iscompared with a measured spectrum to find the overlay error. Inpractice, the parameters such as line widths, thickness, sidewallangles, and overlay errors are typically strongly correlated, and anyincorrect parameter will result in incorrect overlay calculation.

The empirical model-based method compares measured data with anempirical regression line, which approximates the data obtained andanalysed by measuring the diffraction spectrum formed by a series of (atleast four) gratings having different overlay deviation patterns. Thesegratings are fabricated on a wafer. The advantage of this method is thatit does not require a lot of simulations for setting up a matchinglibrary, and therefore there is no strong correlation problem betweenthe parameters. However, one disadvantage of this method is that manygratings must be fabricated and measured on a semiconductor wafer forgathering the library data, which requires a lot of time.

SUMMARY OF THE INVENTION

One aspect of the present invention provides a method for designing anoverlay target, and method and system for measuring an overlay error. Inone aspect, the present invention can eliminate strong correlationproblems among structural parameters. In another aspect, the presentinvention can generate enough theoretical model-based data to reduce thework of fabricating overlay targets on a semiconductor wafer andmeasuring the diffraction spectrum formed by the gratings.

A method for designing an overlay target according to this aspect of thepresent invention comprises selecting a plurality of overlay targetpairs having different overlay errors or offsets, calculating adeviation of the simulated diffraction spectrum for each overlay targetpair, selecting a plurality of sensitive overlay target pairs by takingthe deviation of the simulated diffraction spectrum into consideration,selecting an objective overlay target pair from the sensitive overlaytarget pairs by taking the influence of the structural parameters to thesimulated diffraction spectrum into consideration, and designing theoverlay target pair based on the structural parameter of the objectiveoverlay target pair.

Another aspect of the present invention provides a method for measuringan overlay error comprises measuring a diffraction spectrum of anoverlay target pair, calculating a deviation of the measured diffractionspectrum of the overlay target pair, performing a matching process tosearch a matched deviation in a library, wherein the library stores aplurality of the deviations of the simulated diffraction spectra andcorresponding overlay errors thereof, and determining the overlay errorof the overlay target pair by taking the matched deviation intoconsideration.

Another aspect of the present invention provides a system for measuringan overlay error comprises an angular scatterometer configured toacquire a measured spectrum of an overlay target pair, a data processingmodule configured to calculate the deviation of the measured spectrum ofthe overlay target pair, a library unit configured to store thedeviations of a plurality of simulated spectra and corresponding overlayerrors thereof, and a match unit configured to compare the deviations ofsimulated diffraction spectra with the deviation of the measureddiffraction spectrum of the overlay target pair so as to select anoverlay error of the overlay target pair.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter, which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiment disclosed might be readily utilized as a basisfor modifying or designing other structures or processes for carryingout the same purposes of the present invention. It should also berealized by those skilled in the art that such equivalent constructionsdo not depart from the spirit and scope of the invention as set forth inthe appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The objectives and advantages of the present invention will becomeapparent upon reading the following description and upon reference tothe accompanying drawings in which:

FIG. 1 shows a method for designing an overlay target according to oneembodiment of the present invention;

FIGS. 2A and 2B show an overlay target pair according to one embodimentof the present invention;

FIG. 3 shows a diagram of diffraction light intensity as a function oflight incident angle for overlay targets with different line widthsaccording to one embodiment of the present invention;

FIG. 4 shows a diagram of spectrum difference as a function of overlayerror according to one embodiment of the present invention;

FIG. 5A-5D show diagrams of spectrum difference as a function of overlayerror according to one embodiment of the present invention;

FIG. 6 illustrates a method of measuring an overlay error of an overlaytarget according to one embodiment of the present invention;

FIG. 7 illustrates an angular scatterometer according to one embodimentof the present invention; and

FIG. 8 illustrates a system for measuring overlay error according to oneembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

One primary feature of the present invention is the application of thedeviation of the simulated diffraction spectrum of an overlay targetpair including two overlay targets. In one aspect, the present inventioncan eliminate strong correlation problems among structural parameters.In another aspect, the present invention can generate enough theoreticalmodel-based data and reduce the work of fabricating overlay targets on asemiconductor wafer and measuring the diffraction spectrum formed by theoverlay targets. Therefore, the present invention can have merits ofboth theoretical model-based methods and empirical model-based methods,and avoid the drawbacks of these methods.

FIG. 1 shows a method for designing an overlay target according to oneembodiment of the present invention. The method initially selects aplurality of overlay target pairs having different overlay errors oroffsets. The deviation of the simulated diffraction spectrum formed byeach overlay target pair is then calculated. Next, a plurality ofsensitive overlay target pairs are selected by taking the deviation ofthe simulated diffraction spectrum into consideration. Thereafter, anobjective overlay target pair is selected from the sensitive overlaytarget pairs by taking the influence of structural parameters to thesimulated diffraction spectrum into consideration. Finally, an overlaytarget pair is designed based on the structural parameters of theobjective overlay target pair.

More detailed descriptions of the above steps are provided in FIG. 2-7.The method of the present invention, in summary, obtains the diffractionspectrum formed by an overlay target pair, then generates graphs ofspectrum difference based on the overlay error, and finally sets up amatch library for measuring overlay targets.

FIGS. 2A and 2B show an overlay target pair according to one embodimentof the present invention. The overlay target pair comprises two overlaytargets 20A and 20B. Each overlay target 20A, 20B comprises an uppergrating layer 12, a medium layer 14 and a lower grating layer 16. Bothoverlay targets 20A and 20B have the same structural parameters, but theoverlay errors (d, d+δ) are different. For example, both overlay targets20A and 20B have the same grating pitch, critical dimension line widthand sidewall angle. However, the overlay target 20A has a predeterminedoverlay error (d), and the overlay target 20B has an overlay error,which is equal to the sum of the predetermined overlay error (d) and anoffset (δ). Once the structural parameters, overlay error (d) and offset(δ) of an overlay target pair are determined, the spectrum of theoverlay target pair can be simulated. For instance, the RCWT, a spectrumsimulation algorithm, can be applied to simulate the diffractionspectrum of the overlay target pair.

FIG. 3 shows a diagram of diffraction light intensity as a function oflight incident angle for overlay targets with different line widthsaccording to one embodiment of the present invention. The curves of FIG.3 represent the diffraction light intensities of overlay targets, whichhave grating pitch of 300 nanometers (nm), line width of 100 nm, andoverlay errors of 60, 57 and 90 nm. The horizontal axis shows the lightincident angle, and the vertical axis shows the reflectance. Thedeviation of the diffraction spectrum of each overlay target pair can becalculated by performing difference method between the curve data at acertain angle value. The deviation can be mathematically written infunctional notation as y=ƒ(cd,sa,d,δ), wherein cd is line width, sa issidewall angle, d is an overlay error, and δ is an offset in the overlaytarget pair. In some embodiments, the angle value can be the lightincident angle with the highest level of diffraction intensity, and thedeviation may be a difference value such as the root mean square (RMS)error.

The spectrum difference curves of FIG. 4 can be obtained by changing theoverlay error or offset of an overlay target pair. Each curve representsa function y_(i)=ƒ(cd_(i),sa_(i),d_(i),δ_(i)), which describes thedeviation of the diffraction spectrum of the corresponding overlaytarget pair over the overlay error under the condition of fixed offset(δ). In order to make the most effective use of the information of FIG.4, the segments of these curves where the slope changes abruptly areselected and analyzed in advance. For example, FIG. 4 shows that thecurves have the steepest slopes when the error and offset (representedby (d, δ)) are (195, 60), (135, 150), (150, 135) and (270, 75), and thesteepest slope physically means that the measurement of thecorresponding overlay target pairs can be more sensitive or have higherdiffraction efficiency. Thereafter, the objective overlay target havingminimum structural parameter correlation is selected from the groups ofthe sensitive overlay target pairs, and the overlay target pair can bedesigned based on the structural parameters of the objective overlaytarget pair.

FIG. 5A-5D show diagrams of spectrum difference as a function of overlayerror according to one embodiment of the present invention. The graphsof the spectrum difference shown in FIG. 5A-5D are generated bymeasuring the overlay targets wherein the (d, δ) values are (195, 60),(135, 150), (150, 135) and (270, 75), respectively. The effect ofcritical dimension of line width on the deviation of a simulateddiffraction spectrum can be found from these graphs. In this embodiment,the objective overlay target pair is the overlay target of FIG. 5A,which has a line width of 100 nm, an overlay error of 195 nm and a shiftof 60 nm. If the overlay target pattern of a wafer is designed by usingthe features of the objective overlay target pair, the overlaymeasurement will be more efficient and less influenced by structuralparameters.

In addition, when a library stores the deviation data of the simulateddiffraction spectrum and their corresponding structural parameters, thedeviations of the simulated spectrum stored in the library can be usedto match against the deviation of a measured diffraction spectrum of anoverlay target. Then, the structural parameters of the measured overlaytarget can be found according to the structural parameters of thematched spectrum.

FIG. 6 illustrates a method of measuring an overlay error of an overlaytarget according to one embodiment of the present invention. Thediffraction spectrum of an overlay target pair is initially measured.The deviation of the measured diffraction spectrum of the overlay targetis then calculated. The overlay target pair includes two overlaytargets. Each overlay target comprises an upper grating layer, a mediumlayer, and a lower grating layer. The overlay targets have the samestructural parameters except that the offsets between the upper layerand the lower layer are different. One has the offset of “d,” and theother has “d+δ.” A data-matching process is then performed. Next, adeviation of simulated diffraction spectrum of an overlay target pairmatched to the deviation of the measured diffraction spectrum is foundfrom a library. The library stores the deviations of multiple simulateddiffraction spectra and the corresponding overlay errors thereof.Lastly, the error of the measured overlay target pair can be determinedby the overlay target feature (or error) of the matched deviation of thesimulated diffraction spectrum.

The method of setting up a library initially selects a plurality ofoverlay target pairs having different overlay errors or offsets. Thedeviation of the simulated diffraction spectrum formed by each overlaytarget pair is then calculated. Each overlay target pair includes afirst overlay target and a second overlay target. The calculation methodof the deviation of the simulated diffraction spectrum formed by eachoverlay target includes generating the simulated spectra of the firstoverlay target and the second overlay targets by using a spectrumsimulation algorithm, and then calculating the deviations between thesimulated spectra of the first and second overlay targets. The deviationcan be a difference value, and the difference can be a root mean squareerror. The spectrum simulation algorithm applied to the simulationtechnique of the spectra of the first overlay target and the secondoverlay target includes rigorous coupled wave theory.

FIG. 8 illustrates a system for measuring overlay error according to oneembodiment of the present invention. The system 80 includes an angularscatterometer 82, a data processing module 84, a library unit 86 and amatch unit 88. The library unit 86 is configured to store the deviationsof simulated spectra and the corresponding overlay errors thereof. Theangular scatterometer 82, illustrated in FIG. 7, includes a light source(not shown) directed to an overlay target pair 74 at a light incidentangle 76, and a detector 72 for acquiring measured diffraction spectra.The two overlay targets of an overlay target pair have the samestructural parameters but different offset values. The data processingmodule 84 is used to calculate the deviations of measured diffractionspectrum. The match unit 88 is configured to compare the deviation of ameasured diffraction spectrum with the deviation data stored in alibrary, and then determines the structural parameters and thecorresponding overlay error from the library according to the matcheddeviation data.

The library unit 86 is configured to set up the library. The librarysetup method initially selects a plurality of overlay target pairshaving different overlay errors or offsets. The deviation of thesimulated diffraction spectrum formed by each overlay target pair isthen calculated. Each overlay target pair includes a first overlaytarget and a second overlay target. The calculation method of thedeviation of the simulated diffraction spectrum formed by each overlaytarget includes simulating the spectra of the first and second overlaytargets, and then calculating the deviations of the simulated spectra ofthe first and second overlay targets. The deviation can be a differencevalue, and the difference value can be a root mean squared error. Thetheory applied to simulation technique of the spectra of the firstoverlay target and the second overlay target includes rigorous coupledwave theory.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. For example,many of the processes discussed above can be implemented in differentmethodologies and replaced by other processes, or a combination thereof.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

1. A method for designing an overlay target, comprising the steps of:selecting a plurality of overlay target pairs having different overlayerrors or offsets; calculating a deviation of the simulated diffractionspectrum for each overlay target pair; selecting a plurality ofsensitive overlay target pairs by taking the deviation of the simulateddiffraction spectrum into consideration; selecting an objective overlaytarget pair from the sensitive overlay target pairs by taking theinfluence of the structural parameters to the simulated diffractionspectrum into consideration; and designing the overlay target pair basedon the structural parameter of the objective overlay target pair.
 2. Themethod of claim 1, wherein each of the overlay target pairs comprises afirst overlay target and a second overlay target, and the step ofcalculating a deviation of the simulated diffraction spectrum for eachoverlay target pair comprises the steps of: generating simulated spectraof the first overlay target and the second overlay target by using aspectrum simulation algorithm; and calculating the deviation between thesimulated spectra of the first and second overlay targets.
 3. The methodof claim 2, wherein the deviation is a difference value.
 4. The methodof claim 3, wherein the difference value is a root mean square error. 5.The method of claim 2, wherein the spectrum simulation algorithm isbased on a rigorous coupled wave theory.
 6. The method of claim 1,wherein the step of selecting a plurality of sensitive overlay targetpairs by taking the deviation of the simulated diffraction spectrum intoconsideration further comprises the steps of: generating a graph havinga plurality of spectrum difference curves from the deviation of thesimulated diffraction spectra of the overlay target pairs; and selectinga plurality of sensitive overlay target pairs by analyzing slopes ofspectrum difference curves in the graph.
 7. The method of claim 6,wherein the step of selecting an objective overlay target pair furthercomprises the steps of: changing a predetermined structural parameter ofthe sensitive overlay target pairs; calculating the deviation ofsimulated spectra of the sensitive overlay target pairs with the changedpredetermined structural parameter; and selecting the objective overlaytarget pair by taking a correlation data into consideration, wherein thecorrelation data correlates the deviation of the sensitive overlaytarget pairs and the predetermined structural parameter.
 8. The methodof claim 7, wherein the objective overlay target pair has a minimumcorrelation data.
 9. The method of claim 1, wherein the overlay targetpair comprises a first overlay target and a second overlay target, thefirst overlay target has a predetermined overlay error, and the secondoverlay target has an error equal to the sum of the predeterminedoverlay error and the offset.
 10. A method for measuring an overlayerror, comprising the steps of: measuring a diffraction spectrum of anoverlay target pair; calculating a deviation of the measured diffractionspectrum of the overlay target pair; performing a matching process tosearch a matched deviation in a library, wherein the library stores aplurality of the deviations of the simulated diffraction spectra andcorresponding overlay errors thereof; and determining the overlay errorof the overlay target pair by taking the matched deviation intoconsideration.
 11. The method of claim 10, wherein the overlay targetpair comprises a first overlay target and a second overlay target, andthe calculating step further comprises the steps of: generating measuredspectra of the first overlay target and the second overlay target; andcalculating the deviations of the measured spectra of the first andsecond overlay targets.
 12. The method of claim 10, wherein thedeviation is a difference value.
 13. The method of claim 12, wherein thedifference value is a root mean square error.
 14. The method of claim10, wherein the overlay target pair comprises a first overlay target anda second overlay target, the first overlay target has a predeterminedoverlay error, and the second overlay target has an error equal to thesum of the predetermined overlay error and the offset.
 15. The method ofclaim 10, further comprising the step of defining the library,including: selecting a plurality of the overlay target pairs havingdifferent overlay errors or offsets; and calculating the deviation ofthe simulated diffraction spectrum for each overlay target pair.
 16. Themethod of claim 15, wherein the overlay target pair comprises a firstoverlay target and a second overlay target, and the step of calculatingthe deviation further comprises the steps of: generating simulatedspectra of the first and second overlay targets; and calculating thedeviation between the simulated spectra of the first and second overlaytargets.
 17. The method of claim 16, wherein the deviation is adifference value.
 18. The method of claim 17, wherein the differencevalue is a root mean square error.
 19. The method of claim 16, whereinthe step of generating simulated spectra of the first overlay target andthe second overlay target is based on a rigorous coupled wave theory.20. A system for measuring an overlay error, comprising: an angularscatterometer configured to acquire a measured spectrum of an overlaytarget pair; a data processing module configured to calculate adeviation of the measured spectrum of each overlay target pair; alibrary unit configured to store the deviations of a plurality ofsimulated spectra and corresponding overlay errors thereof; and a matchunit configured to compare the deviations of simulated diffractionspectra with the deviation of the measured diffraction spectrum of theoverlay target pair so as to select an overlay error of the overlaytarget pair.
 21. The system of claim 20, wherein the overlay target paircomprises a first overlay target and a second overlay target, the firstoverlay target has a predetermined overlay error, and the second overlaytarget has an error equal to the sum of the predetermined overlay errorand an offset.
 22. The system of claim 21, wherein the library unit isfurther configured to perform the steps of: selecting a plurality ofoverlay target pairs having different overlay errors or offsets; andcalculating deviations of simulated diffraction spectra of theindividual overlay target pairs.
 23. The system of claim 22, wherein theoverlay target pair comprises a first overlay target and a secondoverlay target and the library unit is further configured to perform thesteps of: generating simulated spectra of the first and second overlaytargets by using a spectrum simulation algorithm; and calculatingdeviations of the simulated spectra of the first and second overlaytargets.
 24. The system of claim 23, wherein the deviation is adifference value.
 25. The system of claim 23, wherein the differencevalue is a root mean square error.
 26. The system of claim 23, whereinthe spectrum simulation algorithm is based on a rigorous coupled wavetheory.